Position tracking and imaging system for use in medical applications

ABSTRACT

A method of monitoring a position of a medical instrument with respect to a patient&#39;s body, including positioning a patient reference unit on a portion of the patient&#39;s body; positioning a remote unit with respect to the medical instrument; associating at least one field sensor with at least one of the patient reference unit and the remote unit; generating an electromagnetic position characteristic field; producing position data responsive to a sensor output signal that is responsive to the presence of the electromagnetic position characteristic field such that the sensor output signal is representative of a position of the remote unit with respect to the patient reference unit; determining image data representing a plurality of images of the patient&#39;s body; and displaying an image of the patient&#39;s body based on the image data and responsive to position image data that varies for different position data.

This is a continuation of Ser. No. 09/211,365, filed Dec. 14, 1998, nowU.S. Pat. No. 6,445,943, issued Sep. 3, 2002, which is a continuation ofSer. No. 08/768,305, filed Dec. 17, 1996, now U.S. Pat. No. 5,967,980,issued Oct. 19, 1999, which is a continuation of Ser. No. 08/306,818,filed Sep. 15, 1994, now U.S. Pat. No. 5,829,444, issued November 3,1998.

BACKGROUND OF THE INVENTION

The invention relates to computer assisted medical surgery and inparticular relates to systems for displaying prerecorded visual imagesduring surgical operations.

Presently available medical imaging techniques such as CAT (ComputerizedAxial Tomography), MRI (Magnetic Resonance Imaging), and PET (PositionEmission Tomography), are known to be helpful not only for diagnosticpurposes, but also for providing assistance during surgery. Prerecordedimages may be displayed during surgical operations to provide thesurgeon with illustrative reference mappings of pertinent portions of apatient's body.

Tracking systems for monitoring the position of a medical instrumenthave also been developed for use with image display systems. Generally,as the surgeon moves the medical instrument with respect to thepatient's body, associated prerecorded images are displayed responsiveto the movement of the instrument. Such tracking systems typicallyinvolve either the use of a passive articulated arm attached to themedical instrument, optical detection or ultrasonic detection.

Tracking systems using a passive articulated mechanical arm attached toa medical instrument are disclosed in U.S. Pat. Nos. 5,186,174 and5,230,623. Generally, as the surgeon moves the surgical instrument withrespect to the patient's body, micro recorders at the joints of thearticulated arm record the respective amounts of movement of each armmember. The outputs of the micro recorders are processed and theposition of the medical instrument with respect to the base of thearticulated arm is thereby monitored. One or more prerecorded images arethen displayed responsive to the movement of the surgical instrument.Such articulated arm tracking systems, however, require that theinstrument be attached to a cumbersome mechanical arm. Also, althoughfree movement of the tip of the arm in three dimensional space may betheoretically possible, the surgeon might experience difficultypositioning the instrument at certain locations and in desiredorientations within the body.

Tracking systems using optical detection (video cameras and/or CCDs(Charge Coupled Devices)) have been proposed for monitoring the positionof a medical instrument with respect to a reference unit as mentioned inU.S. Pat. No. 5,230,623. Such systems, however, require that thereference unit and the instrument both be within the view of the camera.This not only limits the movement of the surgical staff, but alsorequires that at least a portion of the medical instrument remainoutside the patient's body.

Tracking systems using ultrasonic detection are generally disclosed inU.S. Pat. No. 5,230,623. Such systems, however, are disclosed to be usedin a fashion similar to optical detection, i.e., triangulation oftransmitted signals. The transmitted signals are sent from one or moresenders to associated receiver(s), and the distances travelled by thesignals are determined from either timing or amplitude changes. Again,the transmission path must remain unobstructed.

A further shortcoming common to each of the above tracking systems isthat the patient must not move during the operation. Although thepatient is likely to be generally anesthetized, the patient's body maybe inadvertently moved by the surgical staff, or the surgeon may want tomove the body for better positioning. If the body is moved after thetracking system has been initialized, then the tracking will bemisaligned.

There is a need therefore for a system for monitoring the position of amedical instrument with respect to a patient's body that avoids theshortcomings of present devices. Specifically, there is a need for atracking system that permits a medical instrument to be structurallyunattached to a reference unit, yet capable of fully entering into thebody without loss of position monitoring.

There is also a need for a tracking system that monitors the position ofthe patient during surgical operations.

There is also a need for a tracking system that includes a referenceunit that may be easily removed from and accurately repositioned on apatient in precisely the same position. There is further a need for aposition monitoring device that does not obstruct the operating space ofthe surgeon.

SUMMARY OF THE INVENTION

The invention relates to a system for monitoring the position of amedical instrument with respect to a patient's body and for displayingat least one of a plurality of prerecorded images of the body responsiveto the position of the medical instrument. The system includes areference unit, a remote unit, a position characteristic fieldgenerator, a field sensor, a position detection unit and an outputdisplay.

In one embodiment, the reference unit is secured from movement withrespect to at least a portion of the patient's body such that thereference unit is substantially immobile with respect to a targetoperation site. The remote unit is attached to the medical instrument.The field generator is associated with one of the reference or remoteunits and generates a position characteristic field, such as amultiplexed magnetic field, in an area including the target operationsite. The field sensor is associated with the other of the reference orremote units and is responsive to the presence of the field forproducing a sensor output signal representative of the sensed field.

The position detection unit is in communication with the sensor outputsignal and produces position data representative of the position of theremote unit with respect to the reference unit. The output display unitis in communication with the position detection unit for displaying atleast one of the prerecorded images responsive to the position data.

The system further may include a registration unit in communication witha storage unit and the position data. The storage unit stores theplurality of prerecorded images of the body. Each prerecorded image isrepresentative of a planar region within the body such that theplurality of planar regions represented by the prerecorded images definea first coordinate system. The registration unit correlates the positiondata of a second coordinate system (as defined by the position detectionunit) with the plurality of prerecorded images of the first coordinatesystem, and identifies a desired prerecorded image associated with theposition of the remote unit with respect to the patient's body.

The invention also relates to a reference unit that is attachable to apatient's head, and a medical instrument, such as an aspirating device,that is adapted to removably receive a position detection unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the invention may be furtherunderstood with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic view of a system of an embodiment of theinvention;

FIG. 2 is a front view of the headset unit shown in FIG. 1;

FIG. 3 is a side view of the headset unit shown in FIG. 1 taken alongline 3—3 of FIG. 2;

FIG. 4 is a rear view of a portion of the headset shown in FIG. 1 takenalong line 4—4 of FIG. 3;

FIG. 5 is an exploded side view of the surgical instrument and remotesensor shown in FIG. 1;

FIG. 6 is an end view of the assembled surgical instrument and sensorshown in FIG. 1 taken along line 6—6 of FIG. 5;

FIG. 7 is a diagrammatic view of an alternate embodiment of theinvention;

FIGS. 8 and 9 are diagrammatic views of image recording and registrationoperations of the invention; and

FIGS. 10-13 are diagrammatic views of further embodiments of theinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

As shown in FIG. 1, a system 10 of the invention includes a headset 12mounted on a patient 14, a medical instrument 16, a control system 18and a display 20. The control system 18 includes a position detectionunit 22, a registration unit 24, and an image storage unit 26.

The image storage unit 26 stores sets of prerecorded images such as CAT,MRI or PET scan images. Each set of images may be taken along, forexample, coronal, sagittal or axial directions. As shown in FIG. 1, thedisplay 20 shows three images, a coronal image 21 a, a sagittal image 21b, and an axial image 21 c. Text information may also be displayed asshown at 21 d in FIG. 1.

As further shown in FIGS. 2-4, the headset 12 includes two ear mounts 28on side members 30, and a nose bridge mount 32 on a center member 34.The headset 12 should be made of a resilient plastic such that it may besnugly attached to a patient's head, and may be provided in a variety ofsizes. A primary objective of the headset is to provide a reference unitthat may be easily attached to and removed from a patient's head whereinthe headset may be repeatedly reattached in exactly the same place witha high degree of accuracy. In other embodiments, the side members 30 ofthe headset 12 may be rotationally attached to one another and the earmounts 28 may be biased toward one another. Further, the center member34 may be rotatable with respect to the side members 30 and biasedtoward the ear mounts 28 as well.

The headset 12 shown in FIGS. 1-4 also includes a reference unit 36connected to the position detection unit 22 via communication lines 38.The reference unit 36 may be releasably attached to the headset 12 byconventional clamp or fastening means. In one embodiment the referenceunit 36 may include a position characteristic field generator capable ofgenerating a multidirectional field in three dimensions and may involvethe use of either electromagnetic or ultrasonic waves. The positioncharacteristic field differs from the transmit/receive triangulationsystem, in part, because it does not rely on the comparison of onetransmitted signal with another as does triangulation. This permits thepath between the field generator and the remote sensor to be obstructedby materials that do not significantly alter the generated field. Forexample, the position of the medical instrument could be identified evenwhen the instrument is within the patient's body when the generatedfield in a magnetic field. Additionally, the reference unit may alsoinclude a reference sensor 37 to provide verification of proper systemoperation.

In the present embodiment the field generator includes threeorthogonally disposed magnetic dipoles (e.g., current loops orelectromagnets), and the orthogonally disposed magnetic fields generatedby each of the three dipoles are mutually distinguishable from oneanother (e.g., via either phase, frequency, or time divisionmultiplexing). The near-field characteristics of the multiplexedmagnetic fields may be relied upon for position detection, for exampleas generally described in U.S. Pat. No. 4,054,881. Since the presence ofmagnetic material might interfere with the magnetic fields thesematerials are to be avoided in such an electromagnetic system. Inalternate embodiments the field generator may be located somewhere otherthan on the headset and the headset may include two field sensors 36,37.When the distance between the sensors 36,37 is known, the second sensoracts as a backup or reference check for monitoring the proper operationof the system. If the sensed fields are inconsistent then an errorsignal is displayed and/or sounded.

In other embodiments the headset 12 may be employed in systems based onthe triangulation of signals where the reference unit 36 includes one ormore signal transmitters and/or one or more signal receivers. In such atriangulation system, position detection is achieved by comparingcertain characteristics of one transmitted signal with those of a secondtransmitted signal to determine the relative distances travelled. Thetransmitted signals may be electromagnetic (e.g., radio, laser light orlight emitting diodes) or may be ultrasonic. The position of thepatient's head with respect to the surgical instrument may thereby bemonitored.

As shown in FIGS. 5 and 6 the medical instrument 16 may be an aspiratingdevice adapted to removably receive a remote sensor 40 for detecting,for example, the field generated by the position characteristic fieldgenerator. The sensor 40 may be held inside the instrument 16 by forcefit sizing or through the use of a resilient snap member in the wallopening 42. Since an aspirating device is commonly used in most surgicaloperations, incorporating the remote sensor into the aspirating deviceprovides the surgeon with a convenient position detection device thatdoes not clutter the operation site with unnecessary items. Theinstrument 16 may further include a second backup field sensor 41 forsystem error detection as discussed above with reference to the sensor37.

The remote sensors 40,41 are removable from the aspirating device andmay be interchangeably inserted into any of a variety of speciallyadapted surgical instruments. In the illustrated embodiment, the remotesensors 40,41 are received through an opening 42 in the proximal end ofthe instrument 16, and are connected to the position detection unit 22via communication lines 44. The sensors 40,41 may also each includethree orthogonally disposed dipole sensing elements for detecting thepresence of the field generated by the field generator. For example, inone embodiment, the field generator and the sensors each include threeorthogonally disposed electrical wire loops. The generator produces analternating current through one generator loop at a time thus generatinga time division multiplexed alternating electromagnetic field. Thesensor loop signals are each processed in synchronous timing with thegenerator loops to produce outputs responsive to each respectivealternating electromagnetic field.

The distal end of the instrument 16 includes a rigid aspirating tube 46having a flared tip 48. The position of the tip 48 with respect to thecenter of the remote sensor 40 is a known constant and may be easilyseen by the surgeon during surgery. The aspirating tube 46 is in fluidcommunication with an aspirating catheter 50 through the proximal end ofthe instrument 16 via internal channel 52 and a connector element 54.The aspirating catheter 50 (shown in FIG. 1) is connected to a vacuumaspirating unit (not shown).

In operation, the position detection unit monitors the position of themedical instrument 16 with respect to the reference unit 36. Theregistration unit 24 correlates the changes in position of theinstrument 16 with the spacial orientation of the stored images. As thesurgeon moves the medical instrument 16, images appear on the display 20responsive to the position of the medical instrument 16. This permitsthe surgeon to always have available the coronal, sagittal, and axialviews associated with the precise location of the tip 48 of theinstrument 16 regardless of whether the tip 48 is inside of the patient14. Moreover, since the field generator is attached to the patient'shead, the patient is free to be moved without loss of the trackingcapabilities. The display 20 may further identify the location of thetip 48 on each of the displayed images as shown at 56 in FIG. 1. Inother embodiments the orientation of the aspirating tube 46 may also beidentified on the displayed images. In further embodiments, a threedimensional composite image may be displayed based on the prerecordedimages.

As shown in FIG. 7 in an alternate embodiment, the system may include aflexible band 60 for secure attachment to a portion of a patient's body14 (e.g., a head or chest). The band 60 includes field generator 62 anda reference sensor 64 that provides feedback to the signal generator inthe position detection unit 22. The position detection unit 22 isconnected via communication lines 66 to the flexible band 60, and isconnected via communication lines 68 to a flexible medical instrument 70having a remote sensor at its tip 72. Because the medical instrument 70is not rigid, the sensor should be positioned sufficiently close to thetip of the instrument 70 to provide accurate position detection andmonitoring within the patient's body. The display 20 may indicate therelative orientation of the instrument 70 on one or more images asshown.

As illustrated in FIGS. 8 and 9 the registration process involves twofundamental steps: 1) recording the scan images of a predeterminedorientation and 2) mapping the spacial orientation of the positiondetection system onto the recorded images. For example, the orientationsof the prerecorded images may be in the sagittal (y-z plane), coronal(x-z plane) and/or axial (x-y plane) as shown in FIG. 8. The images maybe digitally stored and the distance between each scanned image isrecorded, as are the relative orientations of each set of images.

In one embodiment, fiducial markers 80 are placed on the patient's head14 prior to scanning with the scanner 82. The markers then appear oncertain of the scanned images, and may be located by the positiondetection system as shown in FIG. 9. Specifically, when each marker 80is sequentially located, for example with the tip 48 of a medicalinstrument 16, the user signals the registration unit, such as via acomputer keyboard 84. The registration unit then scans each recordeddigital image beginning from one corner until it locates the identifiedmarker. In other embodiments this may be achieved by having the imagesappear on the display 20 and having the user identify the markers byusing a keyboard or mouse. Once each of the markers have been locatedusing the position detection unit, the registration unit generates amapping function to translate the position detection data (in i-j-kcoordinates) to the stored image orientation data (in x-y-zcoordinates).

In other embodiments, the patient may be wearing an attached referenceunit, such as the headset 12, when the scan images are recorded. Basedon the predefined structure of the reference unit, the registration unitmay then automatically locate portions of the reference unit on thescanned images, thereby identifying the orientation of the referenceunit with respect to the scanned images. Since the relative orientationof the field generator with respect to the reference unit is known, theregistration unit may then generate the appropriate mapping function. Infurther embodiments the surfaces of the patient's skin may be trackedsuch as by a laser light pointer or a movable tip pointer that is biasedin a forward direction. The tracked surfaces may then be located on thestored images. In still further embodiments, the registration unit couldbe programmed to identify characteristic structures or features of thepatient's body and thereby provide fully automatic registration. Forexample, the system might, knowing the size and shape of a headset,identify where the headset would be placed on the patient's head, eventhough it does not appear on the prerecorded images.

As discussed above the position detection system may operate by anydesired principles suitable for generating a field in which positiondetection may be achieved at any location within the field. For example,applicants have found that the 3 Space® Fastrak™ product sold byPolhemus, Incorporated of Colchester, Vermont operates via principlessuitable for use in the present invention. This product uses threeorthogonally disposed coil loops for both the transmitter and thesensors, and produces alternating electromagnetic fields of 8-14 khzthat are time division multiplexed. Those skilled in the art willappreciate that the relative positioning of the field generator and theone or more field sensors is in no way limited to those shown in FIGS. 1and 7.

As shown in FIG. 10, in alternate embodiments of the invention areference unit 86, including a field generator 88, may be positioned asmall distance away from the portion of the patient's body (such as thehead) 14 on an articulated arm 90. A headset 12 including a referencesensor 92 may be attached to the patient's body, and the medicalinstrument 16 may include a remote sensor 40 as discussed above withreference to FIGS. 1-6. Once the field generator 88 is positioned at aconvenient location it may be fixed in place by securing the joints ofthe articulated arm. The position of the patient with respect to thefield generator may accordingly be monitored. The position of theinstrument 16 with respect to the patient may also be determined and thesystem may then operate to display the appropriate prerecorded images asdiscussed above.

In other embodiments, the position of the field generator 88 may beadjusted during the surgical operation by moving the articulated joints.If neither the remote sensor 40 nor the reference sensor 92 are movedwith respect to one another, then moving the field generator 88 shouldnot affect the position detection system. If the accuracy of the systemdepends at all on the relative positions of the field generators 88 andthe sensors 40, 92, then it may be desirable to move the field generator88 during the surgical operation. This may be the case, for example, ifthe system relies on the near-field characteristics of a multiplexedmagnetic field wherein it might be desirable to keep the sensors 40, 92generally equidistant from the generator 88. In still furtherembodiments, the system may periodically prompt the user to repositionthe generator 88 such as through visual cues on the display.

The monitoring of the position of the patient may be accomplished bymeans other than using a headset and reference sensor. For example, acamera 94 connected to an image processor 96 may be positioned to recordthe location of the field generator with respect to the target operationsite of the patient as shown in FIG. 11. If either the patient or thefield generator is moved, the image processor 96 will identify theamount of relative change in location and advise the position detectionunit 22 accordingly. Additional cameras positioned to view the patientfrom a variety of directions may be employed in further embodiments.

As shown in FIGS. 12 and 13 a system of the invention may include aflexible medical instrument 100 having a sensor 102 at its distal tip104, and a fiber optic endoscope 106 having a sensor 108 at it distaltip 110. The fiber optic endoscope 106 is connected at its proximal endto a camera 112 which is in communication with an image processor 114.Because the field generator 62 on the reference band 60 may move, forexample as the patient breaths, the location of the remote sensor 102may appear to move when in fact the medical instrument 100 has notmoved.

To correct for this problem, the fiber optic endoscope 106 can be usedto monitor the position of the tip 104 of the instrument 100 withrespect to the inside of the patient's body as shown. Any sensedmovement of the sensor 102 with respect to the field generator 62 can beevaluated with reference to whether the tip 104 has moved with respectto the interior of the patient's body. If the camera observes that thetip 104 has not moved, but the sensor 102 indicates that it has moved,then the system can identify that such movement was due to the movementof the field generator and not the sensor 102. The system may thenautomatically correct for such variation. Further, the fiber opticendoscope 106 itself may include a sensor 108 for detecting whether thetip 110 of the fiber optic has moved. This should further enhance theaccuracy of the correction system. Also, the camera 112 may providecontinuous registration of the prerecorded images based on the internalstructure of the patient's body.

It will be understood by those skilled in the art that numerousvariations and modifications may be made to the above describedembodiments without departing from the spirit and scope of the presentinvention.

1. A method of monitoring a position of a medical instrument withrespect to a patient's body and for displaying images of the patient'sbody responsive to the position of the medical instrument, said methodcomprising: securing a patient reference unit from movement with respectto the patient's body while permitting the patient's body to be movedwithin an operating environment; securing a remote unit from movementwith respect to the medical instrument; positioning an operatingenvironment reference unit within the operating environment; associatingat least one field generator with at least one of the patient referenceunit, the remote unit and the operating environment reference unit;associating at least one field sensor with at least one of the patientreference unit, the remote unit and the operating environment referenceunit; generating a position characteristic field with the at least onefield generator; producing position data responsive to a sensor outputsignal produced by the at least one field sensor that is responsive tothe presence of the position characteristic field such that the sensoroutput signal is representative of a position of the remote unit withrespect to the patient reference unit; obtaining image data representinga plurality of images of the patient's body; and displaying an image ofthe patient's body based on the image data and responsive to positionimage data that varies for different position data.
 2. The method ofclaim 1, wherein said associating at least one field generator stepcomprises attaching at least one field generator to the patientreference unit; and wherein said associating at least one field sensorstep comprises attaching at least one field sensor to the remote unit.3. The method of claim 1, wherein said associating at least one fieldgenerator step comprises attaching at least one field generator to theoperating environment reference unit; and wherein said associating atleast one field sensor step comprises attaching at least one fieldsensor to the remote unit.
 4. The method of claim 1, further comprisingpositioning a reference sensor a fixed distance from at least one of theat least one field generator and the at least one field sensor.
 5. Themethod of claim 1, further comprising generating a reference outputsignal for use in monitoring the position characteristic field.
 6. Themethod of claim 1, wherein said generating a position characteristicfield comprises generating a position characteristic field that is anon-linear field that emanates from substantially a singular point. 7.The method of claim 1, wherein said generating a position characteristicfield comprises generating a position characteristic filed that is atleast two time multiplexed fields that emanate from substantially thesame point.
 8. The method of claim 1, further comprising registering aspatial orientation of prerecorded scan images with a spatialorientation of the position characteristic field.
 9. The method of claim1, wherein said displaying an image step comprises displaying anindicator representative of a location of the medical instrument withrespect to the patient's body.
 10. A method of monitoring a position ofa medical instrument with respect to a patient's body, said methodcomprising: positioning a patient reference unit on a portion of thepatient's body; positioning a remote unit with respect to the medicalinstrument; associating at least one field sensor with at least one ofthe patient reference unit and the remote unit; generating anelectromagnetic field; producing position data responsive to a sensoroutput signal that is responsive to the presence of the electromagneticfield such that the sensor output signal is representative of a positionof the remote unit with respect to the patient reference unit; obtainingimage data representing a plurality of images of the patient's body; anddisplaying an image of the patient's body based on the image data andresponsive to position image data that varies for different positiondata.
 11. The method of claim 10, wherein said generating anelectromagnetic field step comprises generating an electromagnetic fieldwith at least one field generator associated with at least one of thepatient reference unit and the remote unit, and wherein theelectromagnetic field is capable of passing through the patient's bodyand being substantially undisturbed by passage through the patient'sbody.
 12. The method of claim 10, further comprising positioning anoperating environment reference unit within an operating environment;and wherein said associating at least one field sensor step comprisesassociating at least one field sensor with at least one of the patientreference unit, the remote unit and the operating environment referenceunit.
 13. The method of claim 10, further comprising generating areference output signal for use in monitoring the positioncharacteristic field.
 14. The method of claim 10, wherein saidgenerating an electromagnetic field comprises generating anelectromagnetic field that is a non-linear field that emanates fromsubstantially a singular point.
 15. The method of claim 10, wherein saidgenerating an electromagnetic field comprises generating anelectromagnetic field that is at least two time multiplexed fields thatemanate from substantially the same point.
 16. The method of claim 10,further comprising registering a spatial orientation of prerecorded scanimages with a spatial orientation of the electromagnetic field.
 17. Themethod of claim 10, wherein said displaying an image step comprisesdisplaying an indicator representative of a location of the medicalinstrument with respect to the patient's body.
 18. A method ofmonitoring a position of a medical instrument having a remote unitsecured thereto with respect to a patient's body, said methodcomprising: generating an electromagnetic field; moving the medicalinstrument having the remote unit within the multidirectional field;producing position data responsive to a sensor output signal that isresponsive to the presence of the electromagnetic field such that thesensor output signal is representative of a position of a remote unit,which is secured to the medical instrument, with respect to a referenceunit that is secured to a portion of the patient's body; obtaining imagedata representing a plurality of images of the patient's body; anddisplaying an image of the patient's body based on the image data andresponsive to position image data that varies for different positiondata.
 19. The method of claim 17, wherein said generating anelectromagnetic field step comprises generating an electromagnetic fieldwith a field generator that is secured to a portion of the patient'sbody.
 20. The method of claim 17, wherein said displaying an image stepcomprises displaying an indicator representative of a location of themedical instrument with respect to the patient's body.